Recently, light emitting diodes (LEDs) have been widely used in electronic devices and applications. For example, LEDs have been used as light sources for general illumination. Additionally, LEDs have been used to make display panels, televisions, etc. Regardless of the applications, driving circuits are required to supply power to the LEDs and to control the LEDs to illuminate light with the desired brightness.
An LED display panel generally refers to a device which comprises an array of LEDs that are arranged in one or more rows and columns. Alternatively, an LED display panel may include a plurality of sub-modules, each sub-module having one or more such LED arrays. LED panels may employ arrays of LEDs of a single color or different colors. When LEDs of the same color are used in certain display applications, each LED normally corresponds to a display unit or pixel. When LED panels employ LEDs of different colors, a display unit or pixel normally includes a cluster of three LEDs, which may include a red LED, a green LED, and a blue LED. Such a cluster of three LEDs may be referred to as an RGB unit. Surface mounted RGB units usually have four pins. The first, second, and third pins may respectively correspond to the red, green, and blue LEDs. The fourth pin may correspond to either a common anode or a common cathode of the LEDs.
An LED driving circuit delivers power to the array of LEDs and controls the current delivered to the array of LEDs. The driving circuit may be a single channel driver or a multi-channel driver. Each channel of the driving circuit may deliver power to a plurality of LEDs and control the current delivered to the LEDs. When a group of LEDs is electrically coupled to the same channel, the group of LEDs are often referred to as a “scan line.”
In general, LED driving circuits control the brightness of the LEDs by varying the current delivered to and flowed through the LEDs. In response to the delivered current, the LED emits light with a brightness in accordance with the characteristic specifications of the LED. A greater current delivered to the LED usually translates to a greater intensity of brightness. To effectively control the delivery of current, LED driving circuits may employ a constant current source in combination with the modulation (i.e., turning ON and OFF) of the constant current, using, for example, Pulse Width Modulation (PWM).
FIG. 1A illustrates an ideal PWM signal 110 having a width W and an amplitude A for each PWM cycle. By varying width W of PWM pulse 110, the LED driving circuits may effectively deliver proper driving currents to the LEDs, so as to illuminate light at different shades of gray scale. When delivering the PWM signal, the driving circuit may see different load characteristics for each LED. Such variability of load characteristics may be constituted by a number of accumulating effects, such as the variation of forward voltage Vf for each LED, the variation of intrinsic impedance for each scan line, and the variation of the response to forward current If for each LED. These effects cause huge variations of brightness amongst the LEDs, especially at low gray scale settings. FIG. 1B illustrates an exemplary PWM signal 120 as seen in the driving circuit due to the variability.
Further, at a low gray scale setting, width W of a PWM signal may be sufficiently narrow, such that the pulses of the PWM signal may be lost due to the uncompensated nature of the driving current and the load characteristics. Accordingly, system designers have resorted to a pre-emphasis methodology to overcome the distortion of the PWM signals on PCB traces.
FIG. 1C illustrates an ideal PWM signal 130 with a pre-emphasis portion 135 at the beginning of each PWM cycle. In general, pre-emphasis portion 135 has a duration or width D, which is usually less than width W of a regular PWM cycle, and an amplitude A′ greater than amplitude A of a regular PWM cycle. In the driving circuit for LED display panels, a PWM current signal may be corrected with pre-emphasis, so as to overcome or compensate signal distortions at the rise time. FIG. 1D illustrates an exemplary PWM signal 140 as seen in the driving circuit, with pre-emphasis.
In low gray scale settings, however, duration D of pre-emphasis portion 135 may be proximate or greater than width W of a PWM cycle. Accordingly, even with pre-emphasis, the PWM driving signal may still be distorted, resulting in short pulses and/or accompanying with PWM ringing. Consequently, the PWM driving signal may require further processing, especially for the cases of low gray scale settings.